1. Field of the Invention
This invention relates to a method for the fabrication of a superplastic composite material having metallic aluminum reinforced with silicon nitride whiskers or minute silicon nitride particles by utilizing its own superplasticity.
2. Description of the Prior Art
The conventional ceramic short fiber-reinforced aluminum-based composite material exhibits high specific strength and high specific elastic modulus and excels in abrasion resistance and heat resistance and, therefore, promises extensive utility as a structural material for the aerospace industry. This composite material, however, is deficient in capacity for fabrication.
In view of the deficiency, there have been made various studies on the production of composite materials exhibiting superplasticity.
Typical superplastic composite materials so far developed as the result are: (a) a composite material of 2124 Al alloy reinforced with silicon carbide (SiC) whiskers [T. G. Nieh, C. A. Henshall and J. Wadsworth, "Superplasticity at high strain rates in a SiC whisker reinforced Al alloy" Scripta Metallurgica Vol. 18 (1984 pp 1405-1408], (b) a composite material of 7475 Al alloy reinforced with SiC whiskers [M. W. Mahoney and A. K. Ghosh, "Superplasticity in SiC reinforced Al alloys" Six International Conference on Composite Materials ICCM & ECCM Vol. 2 (1986)], and (c) a composite material of 7064 Al alloy reinforced with minute SiC particles [M. W. Mahoney and A. K. Ghosh, "Superplasticity in a high strength powder aluminum alloy with and without SiC reinforcement" Metallurgical Transactions A Vol. 18A (1987) p 653].
These composite materials are invariably manufactured by a complicated process called a fabrication heat-treatment method (solid-solution treatment aging treatment--hot rolling work--recrystallization treatment).
Further, it has been reported that the transformation superplastic fabrication method (the technique of inducing superplastic deformation by repeating a heating treatment and a cooling treatment alternately) brings about superplastic deformation in (d) a composite material of 6061 Al alloy reinforced with SiC whiskers.
The superplastic material which is produced by this method, however, has a slow deforming speed. The composite material (a) indicated above, when subjected to superplastic deformation, tends to form a liquid phase and give rise to cavities in the metallic Al matrix and, after the superplastic deformation, suffers from deficiency in mechanical properties.
The composite material (b) indicated above is manufactured by a procedure which comprises applying the whiskers to a foil of the 7475 Al alloy and causing the applied whiskers to disperse on and adhere fast to the foil. Thus, it is considered to have dubious stability in mechanical properties.
The present inventors formerly invented a method for the production of a superplastic composite material having metallic aluminum reinforced with silicon nitride and excelling the known superplastic composite materials in practicability, applied this invention for a U.S. patent under Ser. No. 07/497,884 and won the issue of a notice of allowance dated June 20, 1990.